explain chemical equilibria of thymolphthalein?

Question

Using Le Chatelier's Principle, explain what is happening in the test tubes

HInd + OH- <===> Ind- + H2O

Place several drops of thymolphthalein indicator into each of three test tubes.

To the first tube add about 1 mL of 0.1 M hydrochloric acid (HCl).
-No visible change

To the second tube add about 2 mL of 0.1 M ammonium hydroxide (NH4OH).
-blue solution formed, white ppt appeared on standing

To the third tube add about 1 mL of 0.1 M sodium hydroxide (NaOH).
-dark blue solution formed

Split the contents of the second tube in two by pouring about half of its contents into a new test tube. Call these tubes "2a" and "2b."


Add a few drops of 1M hydrochloric acid to tube 2a, and a few drops of 1M ammonium chloride to tube 2b. Record your observations.
2a - blue solution turned colorless, white ppt insoluble in HCl
2b - blue solution turned colorless, white ppt insoluble in HCl

yes, color change is easy to explain, but how about the formation of ppt? what is happening?

Answer 1

HInd has no color, but the anion Ind- has blue color.

When you add HCl solution into the first tube, the HCl dissociates:

HCl + H2O -> H3O+ + Cl-

So, the concentration of H3O+ increases, the concentration of OH- decreases (H3O+ + OH- <=> 2H2O), and the equilibrium shifts to the left - no color.

When you add the weak base NH3 (NH4OH) the concentration of OH- increases:

NH3 + H2O <=> NH4+ + OH-

and the equilibrium shifts to the right - blue color.

The same and more intense when you add the strong base NaOH.

When you add NH4Cl into 2b tube, the NH4Cl dissociates, and the NH4+ reacts with water increasing the [H3O+]

NH4Cl -> NH4+ + Cl-
NH4+ + H2O <=> NH3 + H3O+

This means that the [OH-] decreases, the equilibrium shifts to the right - no color, e.t.c.

Answer 2

Thymolphthalein